The correct answer is C. Paraboloid.
A circular slab subjected to external loading deflects to form a paraboloid because the load causes the slab to bend downward in the center and upward at the edges. This shape is called a paraboloid because it is the surface that is generated by rotating a parabola about its axis.
A semi-hemisphere is a half-sphere, while an ellipsoid is a three-dimensional shape that is similar to an egg. Neither of these shapes is formed when a circular slab is subjected to external loading.